I've used some dialogue from the show in this chapter.

'And you promise you haven't cheated?' Bertie said, teasingly, to Tom as they strode out of the front door of the Abbey to the waiting car.

'As God is my witness, I haven't set eyes on her since yesterday lunch,' Tom replied with a smile. 'I even had breakfast in my room.'

Bertie chuckled. 'That was Carson. He said he won't take any chances with Mary's happiness.'

'That does not surprise me in the slightest,' Tom smiled, knowing how much the butler thought of his intended. 'Thank you for doing this, Bertie, for being my best man. You're a good friend.'

'I rather think you have been a better friend, Tom. I know you're doing this to help me put things right with Edith and I am exceedingly grateful.'

Tom gave his best man a considering look. 'You can thank me by winning her back and making her happy.'

'I intend to if she ever gives me the chance.'

'She can't stay in London forever.' Tom's eyes caught on a car coming up the drive. 'Who's that?'

As the car drew nearer, he broke into a grin as he spotted his errant sister-in-law in the back seat. 'It's Edith! Well, it looks like you might get your chance today after all, Bertie.'

Tom got out of the car, rounding the bonnet to greet Edith as she stepped out of her car. 'Edith! You came! I can't tell you how glad I am.'

'I simply couldn't bear any more telegrams, Tom. I was beginning to hate the sight of the delivery boy,' Edith said, kissing her brother-in-law on the cheek. She looked him up and down. 'You look very handsome.'

'Thank you.'

'Are you sure you want to do this? I don't mind telling you, the news about you and Mary blindsided me. I did not see this coming.'

Tom smiled. 'Neither did your father. But I'm sure, Edith, I'm very sure.'

Edith nodded. 'Then I wish you well. But don't forget that I will always be around to listen when Mary drives you mad. Which we both know she will.'

'And I thank you for that,' Tom answered with a grin.

Edith looked over his shoulder at Bertie standing by the car, a tentative smile on his face. 'Bertie. It's lovely to see you.'

'And you.'

They stood for a few seconds gazing at each other, Tom looking benevolently on, a man happy to see his plan coming together. Then he realised he and Bertie had to get going if he was to stick to Mary's minutely planned wedding timetable.

'I'm sorry, Edith. Bertie and I have to go. Perhaps you can talk later.'

Edith nodded. 'Of course. I need to see Mary anyway.'

'Good luck,' said Tom, leaning in to kiss her cheek again. 'Thank you for coming. It means the world to both of us.'

'Hmm, it might to you, Tom, because you are a darling. I'm not so sure my sister will feel the same.'

'She will. I promise you.'

'We'll see. Now go.'

Cora spun around as the door to Mary's bedroom opened.

'What? I don't believe it!' she cried as Edith came into the room. 'Why didn't you say to expect you?'

'Because I wasn't sure until I got on the train,' Edith replied, taking in the array of surprised faces.

'How are you feeling?' Cora asked, sympathetically.

'Fine,' Edith said, a weary look on her face. 'Can you not ask me that for the rest of the day?'

Mary stared at her sister and then turned to the other women in the room. 'Could you leave us for a moment?'

'Of course,' Cora said, squeezing her younger daughter's shoulders gently as she left the room

'We'll wait for you downstairs,' Rosamund told her newly-arrived niece with a sympathetic smile before following her sister-in-law and Anna out of the room.

Alone, the sisters eyed each other, one wary, the other resigned.

'You know I'm sorry,' Mary said, awkwardly, waiting uncomfortably to see how Edith would react.

'I assumed you would be fairly sorry unless you're actually insane,' Edith said, wandering over to sit in one of Mary's easy chairs.

'Well, I'm not insane. But I am sorry,' Mary said, sitting back down at her dressing table to face her sister. 'I don't know why I did it. Not really.'

'I've told you. Because you were unhappy, so you wanted me to be unhappy too. Now you're happy again, you'll be nicer. For a while.'

Mary stared at Edith, taken aback by her sister's words. 'If that's what you feel, then why are you here?'

Edith regarded her steadily. 'Because, in the end, you're my sister. And one day, only we will remember Sybil. Or Mama or Papa. Or Matthew or Michael. Or Granny or Carson or any of the others who have peopled our youth. Until at last, our shared memories will mean more than our mutual dislike,' she said, her voice calm and resigned.

Mary looked down, digesting Edith's words.

'So, you and Tom,' Edith said, coolly, training careful, considering eyes on Mary.

Mary looked up at her sister, waiting for a cutting remark.

'Is this a sudden thing or has it been going on for a while?'

'It began last year,' Mary admitted. 'But we've only just picked it back up again recently.'

'So, a secret affair then. Or was it just me you kept in the dark about it?'

'No, nobody knew. Well, except Carson, who caught us together once.'

Edith raised an eyebrow. 'Goodness. That must have rocked him to his foundations. His precious Lady Mary up to no good with her brother-in-law. Surely that must have tarnished your halo somewhat?'

Mary nodded. 'He was not pleased with me, but he kept our secret.'

'Of course, he did. He'd do anything for you.' Edith pursed her lips, looking out of the window, before broaching the real question on her mind. 'And did you even think about Sybil when you decided to pursue her husband? Or is this another instance of what Mary wants Mary gets?'

Mary stiffened, feeling her temper rising. She made a valiant effort to tamp it down, knowing she owed Edith some leeway. 'Of course, I thought of Sybil. We both did and we've both had to come to terms with that. And I didn't pursue him. Not at first anyway. If you must know, Tom made the first move. But then it just kind of happened and we fell in love.'

Edith looked at her, contemplatively. 'And you do actually love him, do you? This isn't some kind of jolly jape to you, is it?'

Mary stared at Edith, hurt but not surprised that she should have such a low opinion of her. 'I do. I love him as much as I loved Matthew. It's different, but it's just as strong.'

Edith pinned her sister with a long, hard stare. 'Good. Because if you hurt Tom, I will break with you completely.'

Mary inclined her head, acknowledging Edith's warning. 'You don't need to worry, Edith. Tom has my heart.'

'Hmm, I don't know whether to be pleased for him or worried about him.'

Mary pressed her lips together, swallowing a retort. Instead, she looked over her shoulder at the framed picture of Matthew sitting on her dressing table. 'What do you think he'd have made of it?' she asked. 'I went to his grave to tell him, which isn't like me.'

Edith transferred her gaze to Matthew's face, reminding herself of his good nature and ready smile. 'Matthew loved you. He wanted you to be happy. I'm sure he'd be very, very pleased. In fact, I know he would.'

Mary looked at her sister, for once feeling a level of affection for her. She gave her a small smile, which Edith returned, sharing a rare moment of unity thinking about a man they'd both loved in their different ways.

'You look nice,' Edith said, taking in Mary's bridal outfit.

'Thank you.'

'I'd better go and change then if I'm to make the wedding of the year,' Edith said, rising to her feet.

'Edith, I know I'm the last person you want to talk to about this, but please, speak to Bertie. I think you'll find he has quite a lot to say to you,' Mary said, earnestly, desperate to press the point while she had her sister to herself.

'You're right. I don't want to speak to you about Bertie,' Edith said, sharply.

'Fine, but you should know he's been at Downton for a few days now, and he and Tom have had quite the heart-to-heart about everything.'

Despite herself, Edith slid her sister a curious look, unable to resist. 'Do you really think he might have had a change of heart?'

'I do. He loves you; I know he does. And if he loves you as much as Tom loves me, we will both have the chance to live very happy lives. And don't you think we deserve that?'

Edith quirked her lips in a small, wry smile. 'Well, I certainly do. I'm not so sure about you, sister dear.'

Mary gave her a tentative smile, feeling she might have finally broken through to Edith. 'I know you might not believe this, but I want you to be happy, Edith, and I'm abjectly sorry for the part I've played in preventing that. Like you said, in the end, you are my sister, and you are the only one I have left.'

'Yes, and that is the cross we both have to bear as best we can,' Edith said, dryly. 'I will see you at the church, Mary.'

Mary watched her go, desperately hoping she'd done enough to help salvage Edith's relationship with Bertie.